The enhanced physical downlink control channel (ePDCCH) is a new control channel from 3GPP standard release 11 within the development of the Long Term Evolution (LTE).
A UE is configured with two sets of ePDCCH parameters, each of these sets define a number of hypothesis, control channel message candidates, that the UE must test using blind decodes. Among these parameters are indices to PRBs that the UE should search for ePDCCH in. Other parameters define reference signal sequence.
As illustrated in FIG. 2, ePDCCH is frequency-multiplexed with the Physical Downlink Shared Channel (PDSCH), and the ePDCCH spans both slots in a sub-frame. This means that the whole sub-frame has to be received before the decoding the ePDCCH is possible to perform. The ePDCCH is transmitted in a similar way as PDSCH in transmission mode 9 and 10, i.e. the reception of a message is based on the reception of a demodulation reference signal, DMRS.
Implementing ePDCCH requires a large number of blind decodes in a tight latency budget. The legacy control channel, physical downlink control channel (PDCCH), imposes less tight latency requirements.
If a downlink transport block is scheduled using ePDCCH, decoding of it cannot begin until ePDCCH is decoded. In order to meet the tight latency requirements of the standard it is hence critical that decoding of ePDCCH is as fast as possible.
Decoding of ePDCCH requires testing of several candidates of a large number of parameters. Testing of such a candidates is called a “blind decode”. A UE is required to test up to 32 such blind decodes. Typically a UE will test several hypotheses prior to finding a valid ePDCCH. Even if a valid ePDCCH is detected in a blind decode, UE might be required to continue testing other blind decodes. This is because several ePDCCH might be sent to a given UE in any subframe.
The large number of blind decodes incur an excessive cost in terms of decoding hardware. Also, fewer OFDM symbols of a subframe are needed for PDCCH as compared to ePDCCH such that the UE can enter a power saving mode more often for PDCCH as compared to ePDCCH.
If a demodulation reference signal sequence that the UE is configured to use according to its configured parameter sets is not actually transmitted, the UE will still produce channel estimates, but decoding in subsequent steps will fail. This may be the case if ePDCCH for the UE in question is transmitted using parameters corresponding to another parameter set, or there is no ePDCCH transmitted to the UE in question in this subframe, and there are no other UEs, that have an ePDCCH transmissions in that subframe with the same parameters.